For the integration of photonic circuits/structures into silicon, several building blocks such as modulators, waveguides and detectors are needed. In order to link the electrical domain and the optical domain, devices such as electro-optic modulators need to be developed. In the field of telecommunication, electro-optic materials such as lithium niobate have been used to modulate light at relatively lower power and higher speed. This mature technology has not yet been applicable in silicon-based photonics as silicon does not inherently show any linear electro-optic effect. Also, electro-optic materials that may be integrated into silicon-based photonic structures without the need for specific processing techniques, such as, for example, spin-coating, and/or that may be compatible for the mass fabrication of such devices are not yet generally available. The use of plasma dispersion effects in silicon may alleviate some of the issues limiting silicon-based photonics since materials in addition to silicon may not be needed. However, the exploitation of such effects does not enhance the performance of silicon-based photonic modulators to the extent possible with modulators based on electro-optic materials, which provide an increased bandwidth by facilitating higher order modulation schemes.
It is known that electro-optic active materials can be integrated in waveguide structures that are then used to fabricate electro-optical modulators or switches, such as ring resonators and Mach-Zehnder modulators. An electro-optic active material is a material whose refractive index can be varied by applying an electrical field to this material, such an electrical field hereinafter being referred to as the modulating electrical field. Varying the refractive index of the electro-optic active material via the modulating electrical field can be used to affect the passage of an optical signal/light traversing through the electro-optic active material. The electro-optic effect of an electro-optic active material may depend on certain factors such as crystal orientation, the electrical fields that are applied to the electro-optic active material and also the orientation of light with respect thereto. The extent to which the variation of the refractive index occurs for a given modulating electrical field comprises the electro-optic response of an electro-optic active material.
In respect of electro-optic active materials, it is known that barium titanate has a relatively large associated electro-optic effect and so would be desirable for the basis of an optical modulator. A poling electrical field is applied to barium titanate to align/pole its ferroelectric domains. This is done to be able to record a macroscopic change in the refractive index of the barium titanate when a modulating electrical field is applied thereto. The electro-optic response of barium titanate is increased when the modulating electrical field and poling electrical field are orientated substantially perpendicular with respect to each other.
Reference is now made to US2010/0111303A1 which describes an electro-optic waveguide polarisation modulator comprising a waveguide core having first and second faces defining a waveguide core plane, a plurality of primary electrodes arranged at a first side of the waveguide core plane and out of said plane, and at least one secondary electrode arranged at a second side of the waveguide core plane and out of said plane, wherein the electrodes are adapted in use to provide an electric field having field components in two substantially perpendicular directions within the waveguide core so as to modulate the refractive index thereof such that electromagnetic radiation propagating through the core is converted from a first polarisation state to a second polarisation state. This document discloses a wave-guide core and cladding provided on the core, which separates the core from two top electrodes. The voltages applied to the two top electrodes are used to facilitate an electrical field having a horizontal field component and a vertical field component in the core, which are described as being perpendicular to each other. Consideration is now made as to whether the described configuration would be suitable for when the core comprises high-permittivity and/or electro-optic active materials, such as, for example, barium titanate. Even though the electrical field components may be perpendicular to each other in the present configuration, it is unlikely that the horizontal field component is present at the edges of the core. So, in respect of the core comprising barium titanate, there would be no poling electrical field at the edges of the core and so ferroelectric polarisation of the barium titanate in these regions is unlikely. Furthermore, a relatively large voltage drop occurs in the cladding for voltages applied to the two top electrodes and so the horizontal and/or vertical electrical field components may have relatively low associated field strengths in the core. Thus, the effectiveness of the poling field and/or the modulating electrical field in the electro-optic active material when it comprises barium titanate is expected to be reduced as is its associated electro-optic response.
Turning to U.S. Pat. No. 4,691,984, this document discloses a wavelength-independent electro-optical polarisation mode converter comprising: an electro-optical crystal substrate cut in a plane defined by a direction perpendicular to the optical axis of the crystal; an optical waveguide formed by diffusion of material into a surface of the substrate, the waveguide being oriented to provide for the propagation of light from one end to the other, in a direction parallel with the optical axis of the substrate; and electrode means disposed on the surface of the substrate and positioned with respect to the waveguide to provide control of a coupling coefficient for conversion between one polarisation mode and another, and control of the relative phase between the two modes; whereby both modes experience the same material refractive index, and any phase mismatch between the modes can be corrected by applying a suitable bias voltage through the electrode means. This document discloses a waveguide in which three top electrodes are formed on a waveguide core comprising a lithium niobate layer. A cladding layer is provided between the three top electrodes and the waveguide core. The three top electrodes are configurable to apply a horizontal electrical field and vertical electrical field in the lithium niobate. Because a relatively large voltage drop is likely to occur in the cladding layer when respective voltages are applied to the three top electrodes, the horizontal and/or vertical electrical fields produced in the waveguide core by such voltage application are expected to be of lower field strength, specifically in the lithium niobate layer. Also, a ground electrode is absent in the present configuration so field leakage may occur and particularly the vertical electrical field strength may be further reduced. In respect of if the waveguide core of the present configuration were to comprise barium titanate, the above-discussed factors may cause a reduced efficiency with which it may be poled and/or its refractive index modulated and, therefore, an overall reduced electro-optic response is to be expected.
The document titled, “Low power Mach-Zehnder modulator in silicon-organic hybrid technology”, by Palmer et al. published in IEEE photonics technology letters, vol. 25, no. 13, Jul. 1, 2013, discloses a silicon-organic hybrid modulator based on a Mach-Zehnder interferometer. The device consists of a strip-loaded slot waveguide covered with an electro-optic polymer cladding. This document discloses a slot wave-guide modulator in which a vertical slot filled with electro-optic active materials is provided between silicon block electrodes. A single electrical field in a single direction is disclosed, there is no perpendicular field component. This may pose a limitation for the use of this configuration in a waveguide structure with a core in which the electro-optic active material is, for example, barium titanate, since it cannot be poled perpendicularly with respect to the modulating electrical field and so a reduced electro-optic response is likely.
The document titled, “AlGaAs—GaAs polarisation converter with electro-optic phase mismatch control”, by Grossard et al. published in IEEE photonics technology letters, vol. 13, no. 8, August, 2001, discloses an electro-optic transverse magnetic-transverse electric mode converter with phase mismatch control integrated in AlGaAs—GaAs. Voltages applied to the three electrodes facilitate respective horizontal and vertical electrical fields in the AlGaAs—GaAs layer. This configuration may not be suitable for implementing a waveguide structure in which the core comprises high permittivity and/or electro-optic active materials such as, for example, barium titanate. A relatively large voltage drop is likely to occur in the cladding layer between the three electrodes and the core when respective voltages are applied to the three electrodes. So, it is likely that the horizontal and vertical electrical fields produced in the core by such voltage application are of lower field strength. Furthermore, in the absence of a ground electrode in the present configuration, the vertical electrical field strength is likely to be reduced. In combination, these factors may contribute to a reduced electro-optic response of this configuration.
In the document titled, “A review of lithium niobate modulators for fiber-optic communications systems”, by Wooten et al. published in IEEE journal of selected topics in quantum electronics, vol. 6, issue 1, January-February 2000, a status of the lithium niobate external modulator technology is reviewed. Other waveguide structures and electro-optic device/material technology have been disclosed in patent documents US2012/0148183A1, U.S. Pat. No. 7,224,878B1, EP1271220B1, U.S. Pat. No. 7,224,869B2, U.S. Pat. No. 8,244,076B2, US2004/0114208A1 and in the documents titled, “A strong electro-optically active lead-free ferroelectric integrated on silicon” by Abel et al. published in Nature communications 4, Article no. 1671, April 2013, and “BaTiO3-SrTiO3 multilayer thin film electro-optic waveguide modulator”, by Abel et al. published in Applied Physics Letters, vol. 89, issue 24, December 2006.
Accordingly, it is a challenge to provide a waveguide structure, forming the basis of a silicon-based photonics structure, with an integrated electro-optic active material, that mitigates and/or obviates the drawbacks associated with previously-proposed waveguide structures.